This invention relates generally to an electrophotographic printing machine, and more particularly concerns an apparatus for transferring successive layers of toner particles to the sheet of support material in a prescribed sequence.
In the process of electrophotographic printing, a photoconductive surface is uniformally charged and exposed to a light image of an original document. Exposure of the photoconductive surface creates an electrostatic latent image corresponding to the original document. Toner particles are then deposited on the latent image rendering it visible. Subsequently, the toner powder image is transferred to a sheet of support material and permanently affixed thereto producing a copy of the original document. The foregoing process is described more fully in U.S. Pat. No. 2,297,691 issued to Carlson in 1942.
Multi-color electrophotographic printing is substantially identical to the heretofore discussed process of black and white printing. However, rather than forming a white light image of the original document, the light image is filtered producing a single color light image thereof. The single color light image exposes the charged photoconductive surface to record thereon a single color electrostatic latent image. The single color electrostatic latent image is developed with toner particles of a color complementary to the single color light image. The single color toner powder image is then transferred from the electrostatic latent image to a sheet of support material. This process is repeated a plurality of cycles for differently colored light images and the respective complementarily colored toner particles. Each single color toner powder image is transferred to the sheet of support material in superimposed registration with the prior toner powder image. This creates a multi-layered toner powder image on the sheet of support material. Thereafter, the multi-layered toner powder image is permanently affixed to the sheet of support material creating a color copy corresponding substantially to the colored original document being reproduced.
Hereinbefore, toner powder images have been transferred to the sheet of support material by an electric field created by a corona generator of a type disclosed in U.S. Pat. No. 2,836,725 issued to Vyverberg in 1958. A corona generator of this type induces transfer to the sheet of support material by spraying a corona discharge having a polarity opposite to that of the toner particles on the photoconductive surface. This causes the toner particles to be electrically transferred to the sheet of support material. This type of corona generator has proven to be extremely reliable for transferring a single toner powder image to a sheet of support material. However, such a corona generator does not readily lend itself to transferring multiple toner powder images, in superimposed registration with one another, as is required in color electrophotographic printing.
Other techniques utilized have included an electrically biased transfer roll. The bias transfer roll generates a high voltage discharge in the proximity of the surface of the paper, or it may be applied by means of a conductive cylinder in contact with the paper, as disclosed in U.S. Pat. No. 2,807,233 issued to Fitch in 1957. As taught therein, a sheet of support material is interposed between the conductive roller and surface having the toner powder image thereon. A charge of opposite polarity from the toner particles is deposited on the backside of the sheet of support material which attracts the toner powder image thereto.
Basically, irrespective of the exact mechanism employed to achieve transfer of the toner particles from the latent image, the characteristics of the toner particles and the electrical field determine the quality of the transferred image. In particular, the toner particles have a triboelectric charge thereon and transfer is effected via an electrical field attracting the triboelectrically charaged toner particles from the latent image to the sheet of support material. Many factors influence the quality of the transferred image, the most significant factors being those which affect the uniformity with which the toner powder image is transferred from the photoconductive surface to the sheet of support material. Heretofore, the process of transferring multi-layered toner powder images, as exemplified by a colored electrophotographic printing machine, has produced various problems. In particular, when an electrically biased transfer roll is employed to transfer successive toner powder images, in superimposed registration, to a sheet of support material, hollow characters frequently occur. Hollow characters may be defined as a toner area wherein substantially only the periphery thereof is transferred while the central portion remains devoid of toner particles. The problem of hollow characteristics is most pronounced on line copy reproduction. However, hollow characters frequently occur in solid area copy as well. It has been found that not only does the failure of the toner particles to initially transfer cause the problem of hollow characters, but, frequently, the toner particles back transfer from the sheet of support material to the latent image. This back transfer problem has proven to be one of the most significant causes of poor image quality.
Accordingly, it is a primary object of the present invention to improve transfer in electrophotographic printing by minimizing back transfer of the toner particles from the sheet of support material to the photoconductive surface.